Examples of papermaking belts include a shoe press belt, a calendar belt, a transfer belt, and the like.
General required characteristics for papermaking belts such as a shoe press belt include strength, crack resistance, abrasion resistance, flexibility, and impermeability to water, oil, gas, and the like. Polyurethane, which is obtained by a reaction between a urethane polymer and a curing agent, has been commonly used as a material having these characteristics.
In a papermaking technique, it has been known to form a multiplicity of drain grooves, extending along a travel direction of a wet paper web, in the outer surface of a belt in order to drain water squeezed from the pressed wet paper web. For example, U.S. Pat. No. 4,559,258 describes a papermaking machine belt having such drain grooves.
Japanese Patent No. 2889341 discloses a dehydrating press belt. The dehydrating press belt disclosed in this patent includes a base fabric layer, an intermediate elastic layer formed on at least one surface side of the base fabric layer, a front-surface elastic layer formed outside the intermediate elastic layer, and a back-surface elastic layer formed on the other surface side of the base fabric layer, and is formed by integrally bonding these layers together. The intermediate elastic layer is formed before the front-surface elastic layer in order to remove air remaining in the base fabric layer. In an embodiment disclosed in this patent, the front-surface elastic layer, the intermediate elastic layer, and the back-surface elastic layer are made of polyurethane. Moreover, the shore A hardness of the front-surface elastic layer is higher than that of the back-surface elastic layer, and the shore A hardness of the intermediate elastic layer has an intermediate value between those of the front-surface elastic layer and the back-surface elastic layer. The front-surface elastic layer contains no fiber inside.
Japanese Patent Publication No. H03-75673 of examined applications discloses a blanket for an extended nip press. In the blanket disclosed in this publication, a blanket main body is made of polyurethane having randomly oriented fibers, in order to prevent layer separation and creep of a band-shaped main body while the blanket is in use.
Japanese Patent Publication No. H10-77593 of unexamined applications discloses a blanket with parallel grooves for use in a wide nip press. In the blanket disclosed in this publication, a polyurethane layer is formed on a woven fabric or cotton cloth base formed in a loop. The polyurethane layer has a multiplicity of thin fibers extending in a cross-machine direction. The fibers improve the strength of the polyurethane layer.
In the dehydrating press belt disclosed in Japanese Patent No. 2889341, the front-surface elastic layer is made of polyurethane containing no fiber. Therefore, if cracks are generated, the cracks tend to spread. Moreover, in the case where drain grooves are formed in the front-surface elastic layer, the groove shape cannot be firmly maintained, and the grooves tend to be deformed.
It is possible to strength the polyurethane layer by dispersing fibers in the polyurethane layer, as taught in Japanese Patent Publication No. H03-75673 of examined applications and Japanese Patent Publication No. H10-77593 of unexamined applications.
However, the problem is that containing fibers in the polyurethane layer increases the viscosity, and thus, voids may remain in the base fabric when the base fabric is impregnated with the fiber-containing polyurethane layer.
Moreover, as can be seen in Japanese Patent Publication No. H10-77593 of unexamined applications, orienting the fibers in the cross-machine direction (CD direction) increases the strength difference between the cross machine direction (CD direction) and a machine direction (MD direction), and cracks tend to be generated in the CD direction, and the generated cracks tend to spread in the CD direction.
Moreover, when long fibers are contained in the polyurethane layer, the fibers tend to get tangled each other, making it difficult to uniformly disperse the fibers. A portion where the fibers get tangled becomes a stress concentration point, causing generation of cracks, and the like.